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  • 1
    Publication Date: 2019-07-16
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 2
    Publication Date: 2019-07-16
    Description: Abstract The evolution of the South Atlantic in space and time is in general presented as a two plate system, where the separation of the two continent might be caused by the Tristan Hotspot. Evidence for massive subaerial and submarine volcanism is found on both margins and adjacebt basins. Namely, the Parana (Brasil) and Etendeka (Namibia) flood basalts onshore, and the Walvis Ridge and Rio Grande offshore are evidences for a long term volcanism related to the rift/drift of both continents. In science it is currently under debate, if hotspot volcanism is the driving force for the continental breakup or are e.g. the above mentioned features a by-product of the anyhow moving plates. Aeromagnetic investigations along East Antarctica show that from the beginning of the Gondwana breakup the South American and Africa plates moved in different directions and with different speeds. This most likely caused rifting along old zone of weakness between South America and southern Africa. The ridge in combination with the large igneous provinces (Etendeka and Parana) in South America and Namibia is today considered to be a classical model for hotspot driven continental break-up. For understanding the role of mantle plumes during continental breakup a large-scale geophysical experiment in 2011 on/offshore Namibia was conducted to investigate the crustal/upper mantle structure both under the Walvis Ridge and northern Namibia. Several seismic profiles in the strike and across the Walvis Ridge with in total more than 160 oceanbottom seismometers, magnetic telluric station were deployed. Onshore the number of instruments was more than 300 to map in detail the continent-ocean transition zone to enhance our understanding on how the crust was modified by the thermal anomaly. In addition, a larger number of seismological stations are operated both on- and offshore to investigate the structure of the upper mantle. Here, we reported the first results of this experiment and discuss some implications.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2019-07-16
    Description: The opening of the South Atlantic ocean basin was accompanied by voluminous magmatism on the conjugate continental margins of Africa and South America, including formation of the Parana and Entendeka large igneous provinces (LIP), the build-up of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDR), as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. The area is considered as type example for hotspot or plume-related continental break-up. However, SDR, and LIP features on land are concentrated south of the hotspot tracks. The segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question we address here is whether magmatism is the drives continental break-up, or whether even rifting accompanied by abundant magmatism is in response to crustal and lithospheric stretching governed by large scale plate kinematics. In 2010/11, an amphibious set of wide-angle seismic data was acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The experiments were designed to provide crustal velocity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, constrain the velocity signature of SDR sequences, as well as the extent of magmatic addition to the lower crust near the continent-ocean transition. Sediment cover down to the igneous basement was additionally constrained by reflection seismic data. Here, we present tomographic analysis of the seismic data of one long NNW oriented profile parallel to the continental margin across Walvis Ridge, and a second amphibious profile from the Angola Basin across Walvis Ridge and into the continental interior, crossing the area of the Etendeka Plateau basalts. The most striking feature is the sharp transition in crustal structure and thickness across the northern boundary of Walvis Ridge. Thin oceanic crust (5-7 km) of the Angola Basin lies next to the 35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. Both structures are separated by a very large transform fault zone. The velocity structure of Walvis Ridge lower crust is indicative of gabbro, and, in the lowest parts, of cumulate sequences. On the southern side of Walvis Ridge there is a smooth gradation into the adjacent 25-30 km thick crust underlying the ocean-continent boundary, with a velocity structure resembling that of Walvis Ridge The second profile shows a sharp transition from oceanic to rifted continental crust. The transition zone may be underlain by hydrated uppermost mantle. Below the Etendaka Plateau, an extensive high-velocity body, likely representing gabbros and their cumulates at the base of the crust, indicates magmatic underplating. We summarize by stating that rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magmatic-dominated segment to the south, and an amagmatic segment north of Walvis Ridge.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 4
    Publication Date: 2015-01-18
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 5
    Publication Date: 2015-09-07
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2015-12-18
    Description: Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 7
    Publication Date: 2016-06-19
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 8
    Publication Date: 2014-04-15
    Repository Name: EPIC Alfred Wegener Institut
    Type: Inbook , NonPeerReviewed
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  • 9
    Publication Date: 2019-07-16
    Description: The opening of the South Atlantic ocean basin resulted in voluminous magmatism on the conjugate continental margins of Namibia and Brazil, including the formation of the Parana and Entendeka large igneous provinces (LIPs), the formation of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDRs) near the continent-ocean transition, as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. Hence, the passive margins bordering the South Atlantic are today considered as type examples for models involving hotspot related continental break-up. However,the presence of volcanic features (SDRs, LIPs) appears to be limited south of the hotspot trails. The resulting segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question to be adressed here is whether magmatism is the driving force for continental break-up, or whether even rifting with abundant hotspot related magmatism is in principle in response to crustal and lithospheric stretching. In 2010/11, a combination of on-/offshore wide-angle seismic, marine magnetotelluric and on-/offshore seismological data were acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The set of experiments was designed to provide crustal velocity and conductivity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, the presence of interleaved sediment layers in SDR sequences as well as magmatic intrusions and underplated material near the continent-ocean transition. The sedimentary portions down to the igneous basement were additionally constrained by coincident single-channel reflection seismic data. Here, we present preliminary results for two wide-angle seismic transects and first results for a marine magnetotelluric profile. Tomographic analysis of the seismic data reveals the velocity structure of the crust down into the uppermost mantle. The probably most striking feature of our models is the sharp lateral transition in crustal structure and thickness associated with the northern boundary zone of Walvis Ridge towards the Angola Basin. Here, the rather thin oceanic crust in the basin lies opposite to the ~35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. In contrast, the southern termination of Walvis Ridge and the corresponding transition towards the adjacent 25-30 km thick crustal portions further south is much more subdued. Due to the presence of a high-velocity (6.5-7.2 km/s) lower crust we argue that the Namibian shelf south of Walvis Ridge comprises a transitional igneous origin. We suggest that the northern boundary zone close to the landfall of Walvis Ridge represents an important transtensional tectonic feature which may have provoked the preferential extraction of melts into the footwall of this structure.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 10
    Publication Date: 2015-09-15
    Description: Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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